When it comes to tonally adjusting your image, there are a wide range of options available. Knowing which adjustment to pick can be tricky: do you change the exposure, brightness, levels, lightness? How do they differ? Sometimes there is no absolute answer, and you should do whatever works to your taste. What we can do is evaluate the most commonplace adjustments and get a better understanding of when you might use them.
Brightness & Contrast
One of the most straightforward adjustments you can make to an image is to change its brightness and contrast. Shifting the perceived brightness of an image helps to darken or lighten it, and the shift is typically non-linear, which means it results in a smooth gradual change in brightness. Altering contrast changes the range of tonal values in the image: expanding or increasing contrast results in a ‘punchier’ image with deeper shadow tones and more intense highlight tones.
Use case: Smooth, smaller adjustments to improve the look of an image.
A Curves Adjustment offers an intuitive approach to tonal adjustment: you have a line graph representing the tones of your image (shadows, mid tones and highlights). By adding nodes to this graph, you can manipulate these tones relative to one another. Darkening the shadows, for example, will also gradually bring the highlights down, unless you add another node to boost the highlights: doing so produces a typical ‘S curve’ on the graph, increasing the image’s contrast.
By default, a curves adjustment will apply to all channels in an image. However, separate adjustments for each channel are usually available too, meaning you can determine the tonal mix of your image’s colours.
Altering the exposure in editing software is a simulation of in-camera exposure compensation. It is a linear adjustment measured in ‘stops’ (EV), and modifies all tonal levels equally. It is best used to correct clearly under or over-exposed images rather than for more subtle adjustments, where non-linear adjustments will offer more controllable results.
Use case: Correcting underexposed or overexposed images, extreme adjustments in overall levels.
A typical HSL Adjustment (Hue/Saturation/Lightness) allows you to change and shift the intensity of an image’s colours and its lightness. Most commonly, it is used to increase or decrease saturation, and colour channels can be controlled separately, allowing certain colours to be modified whilst leaving others untouched.
Use case: Adding saturation to an image for a more colourful look.
Using a Levels adjustment typically involves setting the black point, white point and mid-tone (or gamma) range. Combining these controls with a histogram representation of the image allows you to expand or contract the image’s tonal range.
Use case: Stretching and correcting the histogram of an image that is low in contrast, for example foggy/hazy photographs, old scanned photographs.
Shadows and highlights adjustments allow you to isolate and modify those tones separately in an image rather than simply brightening or darkening an image. This means that you can, for example, brighten areas of an image in shadow without affecting the highlights, or vice versa. A common technique is to expose an image for the highlights, then “flatten” its appearance by boosting the shadow tones.
Use case: Brightening up a darkened foreground or interior whilst retaining highlight detail.
Altering an image’s white balance allows you to correct for colour casts or simply influence the mood of the image. An image can be ‘warmed’ or ‘cooled’ by tweaking its colour temperature, and this adjustment can go so far as to dramatically alter the purpose or message of the image.
Use case: Correcting a colour cast, e.g. green/magenta bias.
Tonal adjustments are a brilliant way of improving your images, from subtle corrections that make tones easier on the eye, to more dramatic changes that redefine the image’s look entirely. There are no hard and fast rules as to which adjustments you should use; it’s up to you to decide how to use these adjustments to best modify your image.
Tonal Adjustments and Affinity
All the above adjustments, plus more, are available in the Affinity apps. In addition to being non-destructive, certain adjustments allow cross-colour format editing, meaning you can have LAB-based adjustments in RGB/CMYK documents, and vice versa.